[boost] bug in graph library???

Hello, I'm trying to use the boost graph library and I have a little problem with its implementation of Dijkstra. dijkstra_shortest_paths throws a negative_edge exception and I _do not_ have any negative edge in my graph (checked it both when I add the edges and their weights to the graph and by iterating through the edges once I finish constructing it). Did anyone have a similar problem? Thanks. Andreea